Computed tomographs are used in medical imaging for various tasks in order to obtain images of the interior of a body of an examination object. A computed tomograph includes, inter alia, an X-ray tube, X-ray detectors and a patient positioning table. The X-ray tube and the X-ray detectors are arranged on a rotary frame, the so-called gantry, that rotates during the measurement about the patient positioning table or a system axis, the z-axis, running parallel thereto.
The patient positioning table can be moved in this case relative to the gantry along the system axis. The X-ray tube generates an X-ray beam expanded in a slice plane perpendicular to the system axis in a fan-shaped manner. In the case of examinations in the slice plane, this X-ray bundle penetrates a slice of an object, for example a body slice of a patient who is supported on the patient positioning table, and strikes the X-ray detectors situated opposite the X-ray tube. The angle at which the X-ray beam penetrates the body slice of the patient and, if appropriate, the position of the patient positioning table relative to the gantry vary continuously as a rule during the imaging with the aid of the computed tomograph.
The intensity of the X-rays of the X-ray beam that strike the X-ray detectors after penetrating the patient is a function of the attenuation of the X-rays by the patient. Here, each detector element of a detector row of the X-ray detectors generates as a function of the intensity of the received X-radiation a voltage signal that corresponds to a measurement of the global transparency of the body to X-rays from the X-ray tube to the corresponding X-ray detector element. A set of voltage signals of the detector row that correspond to attenuation data and have been recorded for a special position of the X-ray source relative to the patient is noted as a projection. A set of projections that have been recorded at various positions of the gantry during the rotation of the gantry about the patient is denoted as a scan.
The computed tomograph records many projections at various positions of the X-ray source relative to the body of the patient, in order to reconstruct an image that corresponds to a two-dimensional tomographic image of the body of the patient or a three-dimensional image. In order to record a number of tomographic images or a three-dimensional image, a volume scan is carried out that includes a multiplicity of rotations of the gantry in conjunction with a feed movement of the patient table in the z-direction relative to the gantry. The current method for reconstructing a tomographic image or three-dimensional image from recorded attenuation data is known as the method of filtered backprojection.
For many applications of computed tomography (CT), use has already been made nowadays of multislice computed tomographs that facilitate a better exploitation of the generated X-ray emission, as well as faster 3D X-ray pictures. In the case of these units, the X-ray is also conically expanded in the z-direction and therefore covers a larger object volume per recording position of the gantry. A detector array composed of a number of parallel detector rows is used on the side of the examination volume situated opposite the X-ray tube, and so a number of slices of the object can be recorded in each recording position.
As a rule, the CT examination of vessels or vessel structures requires the injection of a contrast medium in order to be able to distinguish the vessels in the recorded images clearly from the surrounding tissue. The contrast medium must be injected in this case over a sufficiently long time period so that there is a higher proportion of contrast medium in the vessels at the instant of recording or acquiring measured data in each slice recorded during the volume scan.
DE 42 20 282 A1 describes a method for recording a number of two-dimensional X-ray projection images after injection of contrast medium in conjunction with a relative displacement between the examination area and the X-ray recording system. For this purpose, the table plate speed is matched to the propagation speed of the contrast medium so that in the ideal case the contrast medium bolus does not change its position in the individual X-ray pictures. In this regard, the document discloses the determination of the contrast medium speed from two adjacent X-ray pictures, the aim being to determine the position of the contrast medium bolus in the X-ray pictures with the aid of a pattern recognition method.